QUIC: fixed stream cleanup (ticket #2586). Stream connection cleanup handler ngx_quic_stream_cleanup_handler() calls ngx_quic_shutdown_stream() after which it resets the pointer from quic stream to the connection (sc->connection = NULL). Previously if this call failed, sc->connection retained the old value, while the connection was freed by the application code. This resulted later in a second attempt to close the freed connection, which lead to allocator double free error. The fix is to reset the sc->connection pointer in case of error.